Conventionally, as brake systems for a vehicle (automobile), systems provided with a servo unit, such as a negative pressure booster or a hydraulic booster, are known. Further, in recent years, electric servo units using an electric motor as a booster source are known (for example, see Patent Document 1).
The electric servo unit disclosed by Patent Document 1 includes a main piston that is moved forward and backward by operation of a brake pedal, a cylindrical booster piston that is fitted outside the main piston such as to be movable relative to the main piston, and an electric motor for moving this booster piston forward and backward.
By this electric servo unit, the main piston and the booster piston serve as pistons for a master cylinder, and the respective front ends are located in the pressure chamber of the master cylinder. Thus, an input thrust force applied to the main piston from a brake pedal by an operator and a booster thrust force applied from an electric motor to the booster piston can generate a hydraulic brake pressure in the master cylinder.